Such tanks are used, in particular in the nuclear power industry, as storage tanks for storing liquids such as de-gased distilled water.
Very high reliability is required in this industry in the design of tanks, in order to eliminate any risk of operation being stopped, or worse still any deterioration of their essential component parts. It is also essential for operation to be safe without there being any risk of operating error. Furthermore, since such floating roof tanks are used for topping up the primary circuits of a nuclear reactor, it is necessary for the storage to be protected from the air so as to ensure that the stored liquid is of very high quality, for example, in this particular application de-gased water must have an oxygen content of no more than 100 .mu.g/l in order to avoid any risk of oxidation.
Conventionally, such floating roof tanks comprise a cylindrical wall and a floating roof which is surrounded by said wall, and which is connected to it in sealed manner by means of a flexible membrane: a typical example is shown in French Patent Specification No. 2526405. In order to preserve this flexible membrane connecting the periphery of the roof to the side wall of the tank, and in particular in order to avoid kinks forming therein and/or an uncontrolled application thereof against the wall of the tank under the effects of the pressure of the liquid contained in the tank, it is common practice for the space defined by the membrane and the wall of the tank, and in communication with the outside, to receive a counter-pressure liquid which acts essentially as a lubricant for the membrane when the roof moves, and which additionally, at least in some positions, serves to center the said roof relative to the cylindrical wall of the tank.
One of the problems in designing such floating roof tanks lies in controlling the real volume of the counter-pressure liquid, and in particular in emptying this volume.
If this volume is too great, the counter-pressure liquid overflows excessively onto the periphery of the roof, and may drown it when in the low position, which can give rise to a degree of instability and/or to said roof sinking with the consequent risk of damaging the membrane or even the side wall of the tank. If the volume is too small, the membrane runs the risk of sticking to the side wall when the roof is in the high position, and the resulting friction is difficult to control and may damage the membrane.
Further, the volume of the counter-pressure liquid is subjected to the phenomenon of evaporation and this varies depending on location, and on the type of use to which the tank is being put, in other words evaporation varies essentially as a function of temperature and humidity which are diffucult to control accurately. Thus a proposal is made in the above-mentioned French patent specification to cover the surface of the counter-pressure liquid with floating balls or panels in order to reduce evaporation. It should be noted that the side wall of this known tank is provided with an overflow situated at a level below the maximum level reached by the floating roof when in its highest position, but that this overflow cannot under any circumstances be used as emptying means, as can be seen clearly from the high position shown in FIG. 3.
One known technique consists in periodically adding counter-pressure liquid in a highly empirical manner without carefully monitoring the effect of evaporation. In order to have a known real volume of counter-pressure liquid, the tank is completely emptied, the counter-pressure liquid is completely removed, and after these emptying processes, a predetermined volume of counter-pressure liquid is fed into the tank above the roof prior to refilling the tank. This technique has the drawbacks of requiring the tank to be completely emptied, and thus runs the risk of moving the floating roof specifically when its movements may not be properly controlled, and also of a non-negligible loss of operating time which is incompatible with an application in the nuclear power industry.
One proposal which attempts to mitigate these drawbacks consists in emptying the counter-pressure liquid when the roof is in high position via emptying means provided just above the level of the points at which the membrane is fixed to the tank, i.e. half-way up the side wall of the tank. These proposals have not been satisfactory, firstly because the counter-pressure liquid is not completely emptied due to the side wall adhering to a lower zone of the membrane above the emptying means, and secondly since the emptying means are accessible at medium levels, potential users could remove counter-pressure liquid inopportunely, thereby running the risk of its volume being insufficient.
The invention seeks to avoid the above-mentioned drawbacks of the prior art.
One aim of the invention is thus to provide a floating roof tank whose structure ensures that the counter-pressure liquid can be completely removed in an entirely reliable manner when the roof is in its high position.
Another aim of the invention is to provide a tank of simple structure and reasonable manufacturing cost.
Another aim of the invention is to ensure that there is always some liquid stored in the tank, in particular for applications in the nuclear power industry.
A final aim of the invention is to avoid any risk of the counter-pressure liquid being drawn off at inopportune moments.